From Wired How-To Wiki
One of the most-hyped additions to Apple's new iTunes 8 is the
Genius Playlist feature. Pick any song in your iTunes library and ask
the software to recommend additional music you might like using that
original song as a jumping-off point, and you'll get a list of songs
tailored to your tastes.
There are actually two ways to use the Genius feature. One is
the Genius Bar, a sidebar which slides out of the right side of the
iTunes application window and recommends similar songs for sale at the
iTunes Store. The other is the Genius playlist generator, which is
activated by a small button in the lower-right corner of the
application window. The Genius button looks like a small atom, only
without a nucleus -- just the whizzing electrons.
The Genius bar uses collaborative filtering and popularity data
from the iTunes store. The Genius Playlist button uses local meta
information associated with the files, like genre, rating, play count
and other data stored in the ID3 tags.
Here are some tips for getting better Genius recommendations.
This article is a wiki. Got other Genius advice to share? Log in and add it.
Rate your music. Those stars are there for a reason -- use them!
It will give iTunes extra insight about what you really like and what
you don't.
Don't customize genres. You may think Aphex Twin's Come to Daddy
belongs in a genre like "Avant Garde" or "Techno," but the iTunes Store
database (which Genius queries) insists it's "Dance/Electronica."
Change your genres to conflict with iTunes and your recommendations
will suffer. Try to keep your genres matched with what's in the iTunes
Store.
You can also check your genres against what's listed at FreeDB.org.
Search for the artist and album and go to the Details view. Click on
the link to disc's data file and look for the genre -- it will be
listed after the "DGENRE=" parameter.
Watch out for multi-disc albums. iTunes will try to
recommend music it thinks you don't already own. So why does it keep
recommending a song off disc two of that Deep Purple Fireball
remaster even though you already have it? Make sure your tags reflect
the fact both discs 1 and 2 have been imported by entering the
appropriate numbers in the Disc Number fields. Also, are you sure you
imported both discs? Time to double-check.
Deselect poor matches. When iTunes generates a new
Genius playlist and you see a song that you feel doesn't belong,
deselect it by removing the check box next to the song. Then, click
Refresh at the top of the list and the song will be replaced.
It gets better over time. As more users upgrade to
iTunes 8, more libraries are scanned and processed. If your iTunes
Genius is spouting nonsense, try it again in a month.
credit - http://howto.wired.com/wiki/Get_Better_Genius_Recommendations_in_iTunes